Get your elbow on the top of your hip and clench your fist and rest the gun on top of it, Feet need to be shoulders width apart ( you will look a bit of a muppet, but it will greatly improve your standing shoots ) When you have the stance correct if someone gives you a little push you will rock but will come back onto target. It takes quite a bit of practice but is worth the trouble. It is the stance that 10m UIT shooters use.
Breathing is also a big part of good standing shoots, I always breath in, exhale 1/2 my breath as I start to come on to target and hold my breath for the actual shot, and if I feel that dreaded oxygen starvation feeling come on I start the whole cycle again

PS you do not need a sling for this position and a sling will decrease your accuracey.